Conventionally, JP-5-18326A and JP-6-101534A show a fuel vapor treatment system in which fuel vapor generated in a fuel tank is temporarily adsorbed by a canister and a desorbed fuel vapor is purged into an intake passage of an internal combustion engine with air.
Recently, a plug-in hybrid vehicle has been developed. In the plug-in hybrid vehicle, a battery is charged by an external power source while the vehicle is being parked, and the vehicle is driven by almost the electric motor.
In a case that the plug-in hybrid vehicle is driven by only the electric motor, the internal combustion engine seldom works, so that a purge process in which the desorbed fuel vapor is combusted in the internal combustion engine is hardly performed. If a fuel vapor quantity exceeds a fuel vapor adsorbing capacity of the canister, the fuel vapor may be discharged into the atmosphere to cause air pollution. The situation where the fuel vapor quantity exceeds a fuel vapor adsorbing capacity of the canister is referred to as a breakthrough. Besides, if the internal combustion engine is operated frequently to perform the purge processing, the fuel economy will deteriorate.